Stainless steel is made by adding chromium and nickel to iron. This makes the steel magnetic, but not all stainless steel is magnetic. The addition of nickel makes austenitic stainless steel magnetic, while ferritic stainless steel is magnetic without the addition of nickel.
There is a widespread myth that stainless steel is indifferent to the impulses of a magnet. If this were true, why didn’t the X-men discard their armory for a stainless steel basin and cutlery to fight Magneto?
Not only would this strategy fail miserably, but the battle would also look hilarious. Anyway, Magneto would triumph because the myth is not entirely true: stainless steel is magnetic, albeit not all of its five types. So what accounts for this discrepancy?
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What is stainless steel?
Stainless steel is an alloy of iron composed of at least 10.5% chromium. Alloys are metallic materials formed by mixing two metals, typically to enhance the substrate metal’s strength and to impart excellent resistance to corrosion.
The chromium added to a slab of iron forms a thin layer that envelopes the entire surface. This sacrificial layer protects the alloy from corrosion. The severity of protection is proportional to the quantity of chromium. To further strengthen the forces, nickel is added into the mix. To inspire other coveted properties, even manganese, carbon or silicon can be mixed.
What makes a magnet?
Iron, the original metal we begin with, is a ferromagnetic material: it shares a highly genial relationship with magnets. The attraction between a block of iron and a magnet is intense. When attracted to an extremely powerful magnet, the iron can often become inseparable. Magnetic attraction is driven by microscopic magnetic units called ‘domains’.
A domain can be thought of as a miniature compass whose needle points in the direction of a magnetic field. Iron’s crystalline structure comprises an arrangement where a majority of its domains align in the direction of the magnetic field to which it is subjected. What’s more, the imperfections in iron’s structure are fortuitous, as they prohibit the domains from rotating to their original orientation, even after the field is removed. The material then retains its magnetic properties despite being separated from the magnet!
The alignment of domains is analogous to the troupe of fish that Nemo’s dad and Dory consult for directions. If the fish were in disarray, the dense arrow they form wouldn’t be comprehensible. This is the case with paramagnetic or diamagnetic materials, where the alignment of domains in the former is ordered just enough to be magnetized (aluminum), whereas the order is so haphazard in the latter that it is callous to an approaching magnet (silicon).
The operation of domains itself is quantum mechanical; it is, however, irrelevant right now. Yet, if this has stimulated your curiosity, you can find out more here.
It’s all about the alignment
The addition of ‘impurities’, such as chromium or nickel, to iron, is bound to alter its chemical properties. Consequently, the alignment of its domains becomes disturbed. As we feed more and more impurities into it, its magnetic properties change for the worse. Depending on the sprinkled impurities and the resulting crystalline structure, stainless steel can be categorized into five types. However, to evaluate its magnetic properties, we need only two: ferritic and austenitic stainless steel.
Ferritic stainless steel, as the name suggests, contains a fair amount of iron and therefore, naturally, exhibits magnetic properties. Compared to the other types, its carbon content is also meager, which further encourages this magnetic behavior. Popular ferritic materials are automotive parts and some kitchen knives, materials that are iron-chromium binary alloys, containing 13-18% chromium.
On the other hand, austenitic stainless steel is associated with stainless steel. These include kitchen basins and fMRI parts, which comprise iron mixed not only with chromium, but also carbon, nickel and manganese to prevent corrosion in the former and deter error-inducing magnetic attractions in the latter.
The magnetic properties of stainless steel rely on elements added to the alloy, but the addition of nickel is known to accelerate this transition. The presence of nickel alone can transform magnetic stainless steel to non-magnetic stainless steel.
Lastly, because magnetism is merely a function of atomic arrangement, it can also be achieved through structural deformation by thermodynamic means. In simpler terms, subjecting a material to heat can alter, or more precisely, impair its magnetic properties. The heat energizes atoms and drives them haywire, thereby sending them into disarray. Even a ferromagnetic material like iron can exhibit paramagnetic properties after it is heated above a temperature known as the Curie temperature. This temperature barrier varies for different materials.
Conversely, iron’s infatuation with magnets can be intensified by cooling it, transforming it into what is called a super-magnet. Similarly, ferritic stainless steel is purged of its magnetism and austenitic stainless steel attains magnetism when they are heated or chilled, respectively. It is all about the alignment!